Within a small region, or domain, of a thin film of magnetic material, the dipole magnetic moments of neighboring atoms align themselves when placed in an external magnetic field. This alignment of dipole moments is unique to magnetic materials such as Fe, Co, Ni, Gd, and Dy. Despite the random motion undergone by atoms within any material, this magnetic orientation of the atoms in a magnetic material remains even after the externally applied magnetic field is removed.
A transition region exists between adjacent domains of a magnetic material in which the magnetic dipoles have a different direction of alignment. The transition region between two domains with differing alignments is referred to as a domain wall. There are three types of domain walls, each of which is three types of domain walls, each of which is distinguishable from the others in that the magnetic field within each type of domain wall is oriented in a unique way. Within a Neel wall, the transition between the adjacent domains is evidenced by a wall in which the magnetic field rotates within the plane of the thin magnetic film. In a Bloch wall, the magnetic field rotates out of the plane of the magnetic film. The cross-tie wall is the third type of domain wall and reverses the direction of the magnetic field in a small portion of a Neel wall. The cross-tie wall is located between two sections of a Neel wall that have magnetic fields pointing in opposite directions. The section of the Neel wall in which the magnetic field is inverted is bordered by the cross-tie wall on one end and by a Bloch wall or Bloch-line (in a very thin film) on its other end.
In the absence of an external magnetic field of a predetermined strength, the characteristic magnetic field associated with each of the three different types of domain walls will remain unchanged. An external magnetic field of predetermined strength is used to change the magnetic state of a domain wall. These stable domain wall magnetization fields, referred to as domain states, can be utilized for the storage of data in a random access memory system. The resulting device is referred to as a cross-tie memory system. Cross-tie random access memory technology offers the following desirable characteristics: non-volatility, non-destructive read-out, radiation hardness, high density, and a large temperature operating range.
In a cross-tie memory system, there are two stable domain states that are established by application of appropriate magnetic fields to the memory elements in order to store digital data. The domain states are changed by changing non-inverted Neel walls in memory elements to inverted Neel walls and vice-versa, in which an inverted Neel wall is bounded by a cross-tie wall and a Bloch-line. Within a memory element, a Neel wall will separate two large magnetic domains in which the magnetic moments are aligned in exactly opposite directions. On the other hand, creatlon of an inverted Neel wall within the memory element occurs when the direction of the magnetic moments in the surrounding magnetic domains is altered.
U.S. Pat. No. 4,231,107 teaches the significance of the shaping of a thin film of Permalloy magnetic material used to form the memory element, with spaced serrated edges being provided in the strips of magnetic material to give the cross-ties and Bloch-lines preferred locations.
Magnetoresistance is a change in electrical resistance to a flow of current through a memory element due to the application of a magnetic field to the memory element. Read-out of data from a cross-tie memory system is accomplished by the use of magnetoresistive effects. Current is applied to a conductor overlying a memory element and introduces a small magnetic field into the magnetic domains of the memory element. The change in resistance which takes place within the memory element with the application of the small magnetic field is a function of the domain state present in the memory element. Writing of data into the memory element requires the application of a larger magnetic field by means of currents applied to conductors overlying the memory element, and results in the generation of a cross-tie, Bloch-line pair bounding an inverted Neel wall, or the annihilation of a cross-tie, Bloch-line pair.
The read-out of digital data from such cross-tie random access memories utilizes the magnetoresistive effects described above, in which a change in resistance to current flowing through the memory element is produced by applying a magnetic field to the memory element. The amplitude of this change in resistance is a function of whether the memory element is in a state including a non-inverted Neel wall, or a state including an inverted Neel wall with a cross-tie, Bloch-line pair, which are the two stable states used to store binary data.
U.S. Pat. No. 4,473,893 teaches a cross-tie random access memory system with an X-Y array of discrete memory elements 14 formed of conductive striplines 18 of serrated Permalloy film, having aligned rows of conductors 28 overlying the discrete memory elements in the X rows, and serpentine-shaped column-forming conductive striplines 30 aligned with the X row conductors over the individual memory elements of the X-Y memory array. In this cross-tie memory, the digital data is written into memory elements 14 using small amplitude signals applied to both the row and zig-zag conductors. This system includes a row of reference memory elements 16 which are utilized during the read-out operation, with a differential read-out being had between memory elements 14 containing the stored digital data and reference memory elements 16. Each memory element 14 stores a single bit of data. FIGS. 4 and 7 show the voltage or current pulses that are applied during the writing and reading, respectively, of a data bit into a memory element.
The modulation in magnetoresistance produced during memory read-out of a memory element in cross-tie random access memories gives rise to extremely small signals. Due to the small size of the regions containing the domain walls, the resistance changes are minute. The data signals are therefore difficult to detect, especially in the presence of substantial temporal noise. Additional detection problems are caused by offset signals which arise when matching various memory system components, and read problems caused by non-uniform noise.
Re. Pat. No. 30,087, owned by the assignee of the present invention, teaches a coherent sampled CMOS read-out circuit and signal processor coupled to a CCD shift register operated by a two-phase minority carrier transfer clock system. The signal processing taught in this reference is also termed correlated double sampling, which enhances the signal-to-noise ratio of the output signal for the CCD devices. In U.S. Pat. No. 4,035,629, owned by the assignee of the present invention, a correlated double sampling technique is utilized for processing output signals from charge transfer devices. Using this technique, a data signal and a reference level signal are differenced in at least two stages of a read sequence, thereby increasing the probability that the resultant output signal will be correct by reducing the effects of both temporal noise and spatial (due to offset signals) noise. However, neither of these disclosures teaches how correlated double sampling can be applied to a cross-tie random access memory. Furthermore, in some cases it may be desirable to obtain a faster read-out of a single binary digit stored in several memory elements than is achieved when reading in several stages during a read sequence or when reading each memory element one at a time.